The present invention involves the steering systems for vehicles, and more particularly to a steering pull and drift correction system for compensating a vehicle during straight ahead driving.
A vehicle under normal driving conditions can have a drift or pull to one side or the other generated by road crown, wind, alignment, tires and the like. This drift requires a steady adjustment in the steering wheel angle to compensate for this pull and drift to make the vehicle travel straight ahead. This steering wheel adjustment activates the steering system of the vehicle to add a steering force to cancel the pull and drift that makes the vehicle deviate from going straight ahead.
Various types of steering mechanisms have been developed for motor vehicles and the like. Commonly used mechanisms have a hydraulic power source driven by the vehicle's engine. A hydraulic power steering system is an open center system including a pump and reservoir, a flow control valve, a pressure relief valve, a steering control valve and a movable piston inside a power steering cylinder. A hydraulic power steering system utilizes a hydraulically activated piston within the power steering cylinder which is associated with the movement of a rack element of a rack and pinion steering assembly connected to the steerable wheels of the vehicle. Fluid is continually circulated by the pump to the control valve supplying the cylinder before being returned to reservoir portion of the pump and reservoir. When steering assist is required, the driver turns the steering wheel and the power steering control valve diverts fluid to the proper side of the piston. This diversion drives the piston and forces the power steering assembly to turn the wheels of the vehicle; whereby a steering or turning force on the vehicle is produced by way of lateral forces on the tires.
In the patent of Howard (U.S. Pat. No. 4,410,193) an additional cylinder or cylinders are used to accomplish the return of the steering system to a predetermined center position. Howard uses an additional trim cylinder to fine tune the center position of the steering system. Howard discloses other mechanical compensator devices for centering the steering system and returning to a center position in U.S. Pat. Nos. 4,534,577; 5,527,053; and 5,536,028. These devices are concerned with maintaining a center steering position without slack until a steering force from the steering wheel exceeds a preselected value. In U.S. Pat. No. 5,536,028 a trimming unit is provided for varying the center position, including trim chambers on opposite sides of a trim piston. These mechanical components are added to the hydraulic power steering system as well as to tie rods of the vehicle's steering system.
The patents of Yonker (U.S. Pat. No. 4,765,427), Emori et al (U.S. Pat. No. 4,877,100) and Kohata et al (U.S. Pat. No. 5,511,630), all disclose modifications in the power steering gear and housing before it meshes with a rack. In U.S. Pat. No. 5,511,630 the an auxiliary pump system provides stable operation or a power steering system which automatically corrects unexpected variations in the path of a vehicle. The system includes inner and outer sleeves and a torsion bar coupled to the inner sleeve and a piston shaft. A torque motor rotates the torsion bar to provide relative motion between inner and outer sleeves to switch oil passages in a rotary valve which communicate with a power cylinder during unexpected vehicle path variations. The standard steering system is modified by the addition of the torque motor requiring replacement of standard steering components.
The traveled path of the vehicle can be monitored and the steering forces to maintain a desired vehicle trajectory can be controlled. The patent of Frank (U.S. Pat. No. 5,267,627) discloses a power steering controller which monitors vehicle speed and the torque on the steering shaft. A stability augmentation system including an electric motor is responsive to a command signal generated by the power steering controller. The system includes a hydraulic system having a separate oil pump and valve arrangement along with the electric motor controlling the flow of hydraulic fluid to the power steering cylinder. The system is inoperative over a zone based on vehicle speed and output torque of the steering shaft.
The patents of Yasui et al (U.S. Pat. No. 4,941,079 and U.S. Pat. No. 5,313,389) disclose a solenoid valve controller or driver which is directed by a computer using yaw rate, speed and steering wheel angle data from vehicle sensors. Valves are opened or closed to drive a separate added piston inside another cylinder that moves a steering rod of the steering control mechanism. Steering system movements relative to the vehicle are provided in addition to nominal lateral movement provided by the steering mechanism. The steering apparatus and control system provides adaptive control for the moving vehicle to substantially conform the yaw rate of the vehicle to the desired yaw rate. The system maintains vehicle stability as the steerable wheels are independently steered in response to the controller output signal. In a further patent of Yausi et al (U.S. Pat. No. 5,156,229) additional drive components including fluid filled cylinders, housings and electric motors are added to the steering apparatus. These components help to correct the steer angle of the vehicle wheels using controller signals generated from vehicle sensors including vehicle yaw rate sensors.
The need exists to find an economical system for adjusting the steering system of a vehicle to compensate for straight ahead drift without the addition of complex mechanical or electrical components or auxiliary systems. It is known that an internal leak in a power steering system control valve provides a slight vehicle pull forcing the vehicle to drift to one side or the other. This slight force has been found to be adequate to correct for the drift and pull during straight ahead driving.
To overcome the complexity of systems described, one object of the present invention is to find a cost effective means to incorporate a leak in the steering system that can be controlled manually or automatically to correct for a steering pull as needed.
Another object of the present invention is to provide a simple drift control apparatus and for existing vehicles that can easily be adapted to existing hydraulic power steering systems.